1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and a process cartridge, which achieve high durability and high-quality image.
2. Description of the Related Art
Recent developments in information processing systems using an electrophotographic process are remarkable. In particular, laser printers and digital copiers that record information with a laser beam by converting information into digital signals have been remarkably improved in terms of their print quality and reliability. These laser printers and digital copiers have been combined with high-speed technologies. As a result, they have become used as laser printers and digital copiers capable of full-color printing. With the above-mentioned background, as required functions for an electrophotographic photoconductor (hereinafter, may be referred to as “photoconductor”), it is particularly important to satisfy both high-quality image formation and high-durability.
Typically, as photoconductors used for such laser printers and digital copiers and the like using an electrophotographic process, those using an organic photosensitive material are widely used because of the low manufacturing costs, productivity, environmental safety and the like. These organic photoconductors (OPCs) are broadly classified into the following types: for example, (1) photoconductors using a photoconductive resin typified by polyvinyl carbazole (PVK); (2) photoconductors using a charge transporting complex typified by PVK-TNF (2,4,7-trinitrofluorenone); (3) pigment-dispersed type photoconductors using a pigment typified by a phthalocyanine-binder; and (4) function-separated photoconductors each formed with a combination of a charge generating layer containing a charge generating material and a charge transporting layer containing a charge transporting material.
However, an organic photoconductor (OPC) is easily exfoliated from its photosensitive layer in repetitive use. Acceleration of such exfoliation of the photosensitive layer easily causes a reduction in charge potential of the photoconductor, degradation of photosensitivity and further acceleration of background smear due to flaws and defects of the photoconductor surface, a reduction in image density and degradation in image quality. Providing frictional resistance to photoconductors has been a significant conventional issue to achieve. Further, in recent years, smaller diameter of photoconductors resulting from achieving higher-speed performance and down-sizing of image forming apparatuses leads imparting high-durability to photoconductors to a further more significant issue to achieve.
As a method of achieving a highly durable photoconductor, various methods have been widely known, for example, a protective layer is formed as the outermost surface of a photoconductor, and lubricating property is provided to the protective layer, the protective layer is hardened or a filler is added to the protective layer. In particular, the method of adding a filler to a protective layer is one of effective methods to make a photoconductor have high-durability (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 53-133444, 55-157748, 57-30846, 2-4275, 4-281461 and 2000-66434).
However, when a filler is added to the protective layer, the outermost surface of a photoconductor, to obtain high-durability, a residual potential on the photoconductor is likely to increase, and image blur easily occurs due to an oxidizing gas. For achieving high-quality image formation, problems to solve still remain.
Recently, color image forming apparatuses using a roller type charging unit, which has an electric power saving effect, exhibits less ozone generation and allows for achieving its construction capable of downsizing, are mainly used. However, to obtain further higher-durability and higher-speed performance, a corona discharge type charging unit using a non-contact type electrode, which has been conventionally used, is reviewed. However, a corona discharge type charging unit exhibits much more amount of discharge products (e.g., ozone and NOx) generated by discharge electricity than that of a roller type charging unit, and when a photoconductor containing a filler in the outermost surface layer thereof is used to obtain high-durability, the electrical resistance is reduced on the surface of the photoconductor by the discharge products adhered and absorbed to the surface of the photoconductor. Thus, image blur is likely to be caused.
These color image forming apparatuses employ a plurality of image forming units (tandem system) to further achieve high-speed performance. However, when a corona discharge type charging unit is used together, discharge current increases, thereby further increasing the amount of the discharge products (e.g., Ozone and Nox). To reduce the concentration of the discharge products in the atmosphere around the photoconductor, airflow around the charging unit has been controlled, for example (See JP-A No. 2002-296987). However, in view of down-sizing the image forming apparatus, an enough space cannot be obtained for an airflow pathway. Thus, although it has been possible to reduce the average concentration of the discharge products to a certain degree in the vicinity of the charging unit, it has been difficult to uniformly reduce the concentration of the discharge products in the image forming region. Unevenness of the concentration of the discharge products in the image forming region causes unevenness of the halftone image density in accordance with the unevenness of the concentration of the discharge products upon repetitive image forming. The extreme unevenness may cause partial image blur.
Further, to achieve higher-quality of images, an image forming apparatus equipped with a lubricant applying unit which is configured to apply a lubricant over the surface of an electrophotographic photoconductor to reduce the friction coefficient has been proposed in JP-A No. 2003-50477 for the purposes of reducing character dropout and transfer nonuniformity of solid parts and of improving cleaning ability by means of a cleaning blade.
Such an image forming apparatus equipped with a lubricant applying unit has other advantages in that abrasion loss of the surface of the photoconductor and occurrence of photoconductor filming can be reduced, thereby allowing for achieving longer operating life of the photoconductor. Further, when such an image forming apparatus is used in combination with a photoconductor containing a filler at the outermost surface thereof for obtaining higher-durability, abrasion wear caused by a variation in the amount of a lubricant applied by means of the lubricant applying unit can be reduced and the flaw resistance of the photoconductor surface is enhanced, and thus the use of the combination enables further higher durability than in a photoconductor containing no filler at the outermost surface layer.
However, when a lubricant is applied over the surface of an electrophotographic photoconductor, oxidizing gases and oxidizing materials generated from a charging unit and a transferring unit are adsorbed to the lubricant, in addition, when used under a high-humidity condition, the photoconductor surface becomes to have low resistance by the effect of moisture, and image blur may often occur. When the charging unit uses a corona discharge process, oxidizing gases and oxidizing materials adhere or accumulates inside the corona charging unit, and the oxidizing gases and oxidizing materials fall on the photoconductor during stoppage in operation of the photoconductor, and the photoconductor surface with the lubricant applied thereon becomes extremely low resistance, resulting in missing image data and reproducing no image data.
In conclusion, the amount of discharge products (e.g., Ozone, NOx and SOx) generated by the corona discharge type charging unit is greater than that of the roller type charging unit. Further, it is difficult to reduce the unevenness of the concentration of the discharge products in an image forming region in tandem color image forming apparatus. Thus, when the photoconductor, which has the outermost surface layer containing a filler, is employed to achieve high-durability, a latent electrostatic image formed by dots upon repetitive use is slightly moved to the surface and the dots become smaller. As a result, it is likely to cause a reduction in image density and further image blur. Thus, there are not enough effects to the discharge products produced by the corona discharging type charging unit. Therefore, there is still a problem to further enhance high-speed performance, high-durability and high image quality.